1. Field of the Invention
The invention relates to a method and apparatus for treating the surface of the pulpstone of a pulp grinder.
2. Description of Related Art
In general, mechanical pulp is made in piston-loaded pulp grinders, in which the wood material, such as blocks of wood, chips or the like, in pockets thereof are pressed by a loading cylinder and pressure shoe against a pulpstone that rotates longitudinally. In pulp grinders of this kind, the grinding space can be either pressurized or non-pressurized. Another commonly used grinder type is a chain pulp grinder that is characterized by a wood pocket arranged directly above the pulpstone and having two endless chains for feeding the wood against the pulpstone. The chain grinder is a continuous grinder, i.e. wood can be added to the grinder continuously without disturbing the grinding process. To provide the cooling and lubrication required in the grinders and to transport pulp out of them, the pulpstone is sprayed with water. As a result of the grinding effect of the pulpstone and the softening effect of water, the wood fibers detach from the wood material and form, together with water, a fiber pulp suspension.
It is generally known that making mechanical pulp is unstable due to many randomly varying factors. These factors include variations in the quality, size and moisture-content of wood, cleanness of the stone surface, quality of the stone, its surface or cutting pattern, the wear of the grinding surface, and the force pressing the wood against the stone. Non-stability becomes evident in the variation of pulp consistency, quality and fineness. A CSF value is conventionally used as the measure of fineness and correlates quite well with the many quality properties of pulp. The higher the CSF value of pulp, the rougher the pulp.
Even though the pulpstone is significantly harder than the wood fibers, the surface of the pulpstone does wear to some extent during grinding. The surface pattern and roughness of the pulpstone then change and, consequently, the abrasiveness and grinding properties of the stone change correspondingly. As a result, the properties of the formed fibers, and thus also the properties of the fiber pulp suspension, change on the long run, and the usability of the fiber pulp in paper-making, for instance, varies as do the properties of the produced paper. So as to avoid these drawbacks, the pulpstone is reconditioned by “sharpening” it, i.e. by removing material from the surface of the pulpstone to make its properties as desired.
In the prior art, sharpening of the pulpstone is done by moving a bush roll along the surface of the pulpstone and pressing it to the surface of the pulpstone while rotating the pulpstone. As a result of this, material detaches from the surface of the pulpstone over part of the surface, i.e. over the contact area of the bush roll and pulpstone, and this way, by moving the bush roll in the axial direction of the pulpstone, material can be removed from the area of its entire surface, while the pulpstone rotates. With a suitably shaped bush roll, a pulpstone that has become blunt can be reconditioned. This type of solution is known from FI patent 26854, for instance.
A drawback with these known solutions is that during the sharpening stage, the rolls, when turning, not only remove stone material but also break the grinding grains and the edges of the cracked grinding grains become extremely sharp and act almost like a knife. As a result of this, fiber pulp obtained with the pulpstone after its sharpening is sliver-like and contains a great deal of cut short fibers, which reduces the usability of the pulp formed immediately after sharpening. This is why the use of a bush roll is avoided and sharpening is done at relatively long intervals. This, in turn, results in that the CSF value that typically describes the variation of pulp properties varies greatly between two sharpening operations.
In existing commercial systems, the quality and production control of pulp grinders is based on what is known as target range control. According to target range control, quite a large operating range is allowed for an individual pulp grinder both in ground pulp quality and in pulpstone sharpness. The reason for this procedure is in the pulpstone surface treatment technique using steel rolls. Roll sharpening causes quite a big change in quality after the treatment that needs to be compensated for by altering the production speed or grinding power. Many earlier control systems are based on models, in which the change of the pulpstone surface on the long run is predicted using a computational sharpness of the pulpstone. The quality of pulp with a specific pulpstone sharpness is, in turn, predicted with a CSF model, in which the descriptor is not only the sharpness of the pulpstone, but also the grinding power or production speed. “Tavio, P., Korhonen, J.: AGMO—Automated Groundwood Mill Operator, Pulp Paper Mag. Can. 75 (1974), pages T 268 to T 272”; Kallioniemi, J.: Kokemuksia tietokonepohjaisesta hiomon ohjauksesta (Experience in computer-based groundwood mill control), Automaatiopäivät 1984, publication 10, volume II, publisher Suomen Säätötekninen Seura, pages 123 to 136”; and “Kärnä, A., Liimatainen, H.: Control of pressurized grinding: Initial experiences at Anjala, Pulp Paper Can. 86 (1985) 12, pages T 377 to T383” describe the above-mentioned control systems.
U.S. Pat. No. 5,727,992 describes a method for sharpening a pulpstone with a high-pressure water jet. The sharpening is done with equipment comprising at least one nozzle that is connected to move in the axial direction of the pulpstone during sharpening in such a manner that the entire width of the pulpstone is treated by the sharpening water jet sprayed from the nozzle. A pressure pump is connected to the nozzle to pump a high-pressure water jet through the nozzle against the surface of the pulpstone while the pulpstone is rotated during sharpening.
This water sharpening technique allows for a more controlled treatment of the pulpstone surface than the roll sharpening, and the compensation of the quality change in pulp by altering the production speed or grinding power of the pulp grinder is almost unnecessary. In addition, the technique makes it possible to have the same quality target for all pulp grinders and the target range principle can be dropped. Further, the publication states that the CSF value of pulp is monitored essentially continuously and water sharpening is started when the CSF value reaches a predefined low limit, and water sharpening is stopped when the CSF value reaches a predefined high limit.
WO publication 00/73571 describes a similar method for sharpening a pulpstone as in the U.S. Pat. No. 5,727,992, in which an optimization algorithm is added to the method. This publication also emphasizes that in water sharpening, the treatment pressure of the pulpstone can be raised during the treatment.
A drawback with the arrangements described above is, however, that they assume that the quality of pulp is monitored during water sharpening and water sharpening is stopped when the quality of pulp differs from the target to a certain extent. A further problem with the arrangements is that the quality of pulp cannot be measured very quickly in practice, especially if tearing strength is used as a control criterion. Thus, the speed of determining the quality of pulp affects the degree of sharpness provided for the pulpstone during this time.